1. Field of the Invention
The present invention relates to high-pressure discharge lamps including a metal halide lamp, an ultra high-pressure mercury lamp and the like, and more particularly to a high-pressure discharge lamp which operates in a condition close to a point source.
2. Description of the Related Art
FIG. 1 illustrates prior art direct-current high-pressure discharge lamp 60 which is described below. A pair of opposing tungsten electrodes 62A, 62B are inserted into lamp tube 61 made of quartz glass. In direct-current high-pressure discharge lamp 60 illustrated in FIG. 1, both tungsten electrodes 62A, 62B have different shapes. In consideration of tungsten which becomes worn due to evaporation during a discharge, tungsten electrode 62A, which acts as an anode in the operation of the discharge lamp, is designed to have larger dimensions than tungsten electrode 62B which acts as a cathode. Tungsten electrodes 62A, 62B are made of highly pure tungsten having the purity of 99.99% or higher. Each of plural iron-group metals contained in the tungsten merely has a content of 10 ppm or less, and even a total of their contents amounts to 10 ppm or less.
Molybdenum rods 64A, 64B are connected to tungsten electrodes 62A, 62B, respectively, through molybdenum foils (Mo foils) 63A, 63B for serving as electric lead wires. Such electrode parts are inserted respectively from insert ports 65A, 65B of lamp tube 61 at both ends thereof, and a rear end of each tungsten electrode 62A, 62B, molybdenum foils (Mo foils) 63A, 63B and one end of each molybdenum rod 64A, 64B are embedded in both end portions of lamp tube 61 with quartz glass. In this way, insert ports 65A, 65B are closed to hermetically seal the interior of lamp tube 61. Hermetically sealed lamp tube 61 is filled with mercury, a halogen gas, and an inert gas after it is evacuated to a high vacuum.
High-pressure discharge lamps as described above may be used for lamp light sources for data projectors represented by a liquid crystal projector, and rear projection televisions. Generally, a lamp for such a projector comprises a reflector which has a parabolic surface for collimating light from a light source and impinging the collimated light onto an optical system. In this configuration, when the light source positioned at the focal point of the parabolic surface of the reflector is a point source, a parallel beam is radiated from an opening of the reflector, so that the light can be efficiently directed to an optical system of the projector on which it is subsequently incident. However, since actual light sources are not ideal point sources but have a certain size, light reflected from the reflector has an expanse, resulting in an incident efficiency to the optical system which is lower than that of an ideal point source. With recent proliferation of projectors, the projectors are required to improve the incident efficiency to the optical system. For this purpose, a so-called short arc lamp is desired for a high-pressure discharge lamp for use as a light source which is required to have a short distance between electrodes and a small discharge plasma to substantially provide a point source condition, in order to improve the incident efficiency to the optical system.
Therefore, when the conventional high-pressure discharge lamp illustrated in FIG. 1 is used in a projector, the inter-electrode distance between the leading ends of both tungsten electrodes 62A and 62B is set to be approximately 1 to 2 mm or less to reduce the length of a discharge plasma, and the ends of the electrodes are designed to have a conical shape to reduce the diameter of the discharge plasma, in order to provide the “short arc” lamp.
However, when the conventional high-pressure discharge lamp is operated for a long time, the following problems occur. FIG. 2 illustrates the shape of the anode after the conventional direct-current high-pressure discharge lamp was operated for 2,000 hours. As illustrated, when the conventional direct-current high-pressure discharge lamp is operated for a long time, the conical leading end of tungsten electrode 62A becomes worn so as to be flat, resulting in a significant increase in the inter-electrode distance. Further, an increasingly blunt angle at the leading end of the anode causes the discharge plasma to radially expand, in contrast to the point source, resulting in a lower incident efficiency to the optical system.
As a known document, JP-A-2001-319617 shows an improvement on the purity of tungsten which is a material for electrodes, wherein an Fe content in a tungsten electrode is desirably 3 ppm or less. In other words, JP-A-2001-319617 proposes a reduction of Fe as impurities for the tungsten electrode.
However, even with these proposals, it is difficult to maintain for a long time the “short arc” condition with a short inter-electrode distance and a small discharge plasma, as is similar to the conventional high-pressure discharge lamp described above. The leading ends of the electrodes are worn to cause a change in shape. Consequently, a projector designed for a “short arc” high-pressure discharge lamp suffers from a significantly reduced incident efficiency to an optical system and a problem of a short product life.